This invention relates to a braking force control device for two-circuit brake systems, in which a first brake circuit is directly connected to the master brake cylinder without any change in pressure and a second brake circuit is connected to the master brake cylinder via a pressure modulating element of the control device. The brake pressure in the second brake circuit is changed in a relation determined by a controlling piston after a switching over pressure determined by the end surfaces of the controlling piston and the force of a control spring acting on the controlling piston has been exceeded. A locking piston operated hydraulically is incorporated in the control device for cancelling or locking the change in pressure in the controlled brake circuit when the uncontrolled brake circuit fails.
The braking force control device may be designed as single-stage or continuously working braking force device. The single-stage braking force control devices, which limit the pressure of the uncontrolled brake circuit to a certain value, are in general called braking force limiting devices. In these braking force limiting devices only one end surface of the control piston is applied with pressure, e.g. by the controlled pressure. A spring, which together with the surface applied with the pressure determines the switching over pressure, acts against the force produced by the controlled pressure. In the brake force control devices working continuously a second end surface is also applied with pressure, that is by the pressure to be controlled. The force produced by the pressure to be controlled acts in the same direction as the spring force.
An improved braking force control device having a short overall length is disclosed in copending U.S. Application Ser. No. 135,580, filed Mar. 31, 1980, assigned to the same assignee as the instant application, and whose disclosures is incorporated herein by reference. This control device is seated in a multiple-step housing which is closed by a plug. This plug has a coaxial channel for the pressure medium to be controlled.